The invention relates generally to radar receiver attenuation, and more particularly to a system for adaptively adjusting the radar receiver attenuation of input signals having a wide dynamic range to maintain the attenuated signals within a predetermined range for radar post processing operations.
In general, radar systems deal with input signals that have a wide dynamic range. These signals are normally scaled with one or more attenuators in the radar receiver so that post processing in a digital signal processor, for example, may be accomplished with signals within a predetermined desired range. The receiver attenuation may be dynamically adjusted to the input signaling by means of an automatic gain control, AGC, loop to maintain the attenuated signals within the desired range of the digital signal processor. A desirable AGC loop design attempts to minimize the responsiveness of the radar receiver to rapidly changing input signal strength which may cause either saturation or, so much attenuation, that a target may go undetected (i.e., radar blind time). However, along the same lines, the AGC loop should not be designed to respond to rapid fluctuations of noisy input signals.
A typical radar receiver including an AGC loop may be embodied similar to that shown by the functional block diagram of FIG. 1. Referring to FIG. 1, the radar receiver 10 may include an RF stage 12 and one or more IF stages 14 yielding a video signal 16 which may have a frequency spectrum surrounding a frequency at or near baseband level. Each IF stage 14 may include a conventional combination of a mixer 18 and a filtering/conditioning amplifier 20. The mixer 18 is generally used to beat the frequency of the RF signals down to a lower level utilizing a reference frequency signal 22 to generate a signal 24 which is then filtered and conditioned by the conventional circuit 20. The radar receiver 10 generally includes conventional receiver attenuation for attenuating the input signaling flowing therethrough. This receiver attenuation may be found in either the RF stage 12, IF stages 14 or a combination thereof and is normally calibrated logarithmically, like in units of decibels, for example. The receiver attenuation may be governed by one or more AGC signals 26.
In most modern radars, the video signal 16 may be digitized by a high speed analog-to-digital (A/D) converter 28 with the resulting signals provided to a post processor 30 which may be a digital signal processor, for example. In addition, the digitized attenuated input signals may be provided to a clutter analyzer 32 which derives a signal representative of the clutter content of the input signals receiver over the time interval of a radar look. The derived signals 34 may be provided to a conventional radar computer 36 which may be programmed to perform the functions of the AGC loop which generates the signaling 26.
A typical operation of the AGC loop embodiment of FIG. 1 may be described in connection with the exemplary waveforms 2A-2C of FIG. 2. Referring to FIG. 2, during an initial radar loop L0 from time t0 to t1, the receiver 10 is attenuating the received input signals 11 with an initial receiver attenuation and providing the attenuated signals via A/D converter 28 to the clutter analyzer 32. The clutter content is derived in the clutter analyzer 32 during a time interval 40, for example, as shown in the waveform 2B subsequent to the time t1. The derived signal is thereafter provided to the radar computer 36 which generates therefrom the adjustment signal 26 for the receiver attenuation. The receiver attenuation may be readjusted by the newly derived adjustment signal 26 during the look L2 as designated by the update pulse 42 at time t2. The time periods for the looks of the radars are designated by .tau.. Thus, in the present example, there exists a 2.tau. delay in the adjustment of the receiver attenuation with regard to the corresponding input signaling used to derive the adjustment signal 26 of time interval 40. The above process may be continued similarly for subsequent looks L1, L2, L3, . . . wherein the input signaling is collected during the corresponding radar look resulting in a delayed derivation of the respective adjustment signal 26 by one radar look in each case as designated by the time intervals 44, 46, . . . in waveform 2B. Likewise, the radar computer 36 provides the derived adjustment signals to adjust the receiver attenuation respectively at time intervals designated by 48, 50, . . . in waveform 2C.
One drawback of the present AGC loop is that the derivation of the clutter measurement of a radar look in the clutter analyzer 32 is not compatible with the logarithmic calibration of the receiver attenuation. That is, the clutter measurement signal is non-linear with respect to the receiver attenuation calibration, and vice versa. For example, a 6 dB decrease in receiver attenuation doubles the attenuated input signal level as measured at the output of the A/D converter 28, for example. On the other hand, a 6 dB increase in receiver attenuation control reduces the measured input signal level in half. Looking at this numerically, in the former case, the digitized signal level may double from 500 to 1,000, for example, resulting in a difference of 500. However in the latter case, the signal level reduces from 500 to 250, yielding a difference of only 250. Both of these values result from the same logarithmic adjustment value in dB's for the receiver attenuation.
The non-linear characteristics of the AGC loop in combination with its 2nd order delay operation provides for a number of undesirable effects. For example, the receiver attenuation adjustment signal 26 has a tendency to overshoot the desired correction for a rapidly increasing received input signal. An illustration of this response is shown in the graph of FIG. 3 in which the received input signaling, depicted by the line 54, increases rapidly by 20 dBs starting from the radar look L0. The computed AGC signal 26, as depicted by the solid line 56 in the graph of FIG. 3, begins responding after two radar looks and appreciably overshoots the desired correction level 58 before eventually converging to the desired level 58 over a longer than desired time interval. Another undesirable effect occurs for a rapidly decreasing received input signal such as that shown by the solid line 60 in FIG. 3, for example, in response to which the AGC signal 26 decays very slowly to the desired lower value 62 as shown by the solid line 64. One stumbling block to alleviating these effects is that the transient performance for a non-linear AGC loop is often very difficult to analyze.
Nonetheless, because of the importance given to increasing the probability of target detection in radars, it is felt necessary to further minimize the response time of the AGC loop to rapidly changing input signals in order to reduce the blind time of the radar. However, at the same time, it is also necessary to maintain the filtering capabilities of the AGC loop so that it does not respond adversely to rapid fluctuations of a noisy input signal.